The present invention relates to a novel oxazolidone ring-containing epoxy resin, more specifically, relates to an oxazolidone ring-containing epoxy resin which is preferred to be used as a resin for an aqueous coating composition.
An epoxy resin is the resin which has more than two epoxy groups (xe2x80x94CH(O)CH2) in the molecule. An epoxy resin affords a cured resin excellent in dielectric ability, mechanical strength, dimensional stability, and chemical resistance when it is cured by combining a suitable curing agent. An epoxy resin therefore has variety of applications such as a paint for metal, an anticorrosion material, an adhesive, a structural material reinforced by glass fibers, and the like.
J. Polymer Sci. Part A-1, 4, 751 (1966), Iwakura et al, describes an oxazolidone ring-containing epoxy resin. The oxazolidone ring-containing epoxy resin is that obtained by allowing a blocked diisocyanate compound which is obtained by reaction of diisocyanate and monoalcohol, to react with a diepoxy compound. J. Appl. Polymer Sci., 9, 1984 (1966), Sander et al, for example describes an oxazolidone ring-containing epoxy resin which is obtained by allowing a diisocyanate compound to directly react with a diepoxy compound.
The epoxy resins may be employed as a binder resin for an aqueous coating composition. In this instance, an ionic group is introduced in the epoxy resins to provide hydrophilic epoxy resins. Japanese Patent Laid Open Publication No. 306327/1993 describes the method of that a terminal epoxy group of the oxazolidone ring-containing epoxy resin prepared by the above described means, is ring-opened with an active hydrogen compound having an ionic group to prepare an oxazolidone ring-containing aqueous resin having an ionic group such as an amino group, and a carboxyl group therein. This publication also describes an aqueous coating composition which comprises such an oxazolidone ring-containing epoxy resin.
Corrosion resistance, impact resistance, cohesiveness, excellent appearance, and the like are generally required for a resin for use in a coated film. An oxazolidone ring-containing epoxy resin is excellent in heat resistance, and corrosion resistance, and therefore affords a coated film excellent in heat resistance, and corrosion resistance. However, the conventional oxazolidone ring-containing epoxy resin is poor in flexibility. Therefore, impact resistance of a coated film becomes poor when the oxazolidone ring-containing epoxy resin is used as a resin for a coated film.
The present invention provides a novel oxazolidone ring-containing epoxy resin suitable for use as a resin for a coated film. The oxazolidone ring-containing epoxy resin of the present invention affords a coated film excellent in impact resistance in addition to heat resistance, and corrosion resistance.
The oxazolidone ring-containing epoxy resin has a structure of the formula: 
wherein R3 represents a residue excluding epoxy groups of diepoxide, X represents a residue excluding isocyanate groups of polyurethane diisocyanate, and n represents an integer of 1 to 5; and wherein X has a structure of the formula: 
wherein R1 represents a residue excluding isocyanate groups of diisocyanate, R2 represents a residue excluding hydroxyl groups of diol, and m represents an integer of 2 to 10.
The oxazolidone ring-containing epoxy resin of the present invention is preferably prepared by the process comprising the steps of:
obtaining a blocked polyurethane diisocyanate represented by the formula: 
xe2x80x83wherein R1 represents a residue excluding isocyanate groups of diisocyanate, R2 represents a residue excluding hydroxyl groups of diol, B represents a residue of a blocking agent, and m represents an integer of 2 to 10, by reacting diisocyanate, diol, and a blocking agent; and
allowing the blocked polyurethane diisocyanate to react with diepoxide.
Diisocyanate means a compound which has two isocyanate groups in the molecule. Specific examples of the diisocyanate include aromatic diisocyanate such as 4,4xe2x80x2-diphenylmethane diisocyanate (MDI), tolylene diisocyanate (TDI), and xylylene diisocyanate (XDI); aliphatic diisocyanate (comprising alicyclic diisocyanate) such as hexamethylene diisocyanate (HMDI), isophorone diisocyanate (IPDI), 4,4xe2x80x2-methylenebis(cyclohexylisocyanate), trimethyl hexamethylene diisocyanate.
Preferred diisocyanate is the aromatic diisocyanate. The aromatic diisocyanate is highly reactive with an epoxy group present in an epoxy resin, and an oxazolidone ring may easily be formed.
Diol means a compound which has two hydroxyl groups in the molecule. For example, oligomeric diol or polymeric diol preferably having a molecular weight of from 300 to 9000 is comprised in the diol of the present invention. Throughout the specification and claims, the wording xe2x80x9cmolecular weightxe2x80x9d means number average molecular weight.
Specific examples of the diol include alkylene diol, aromatic diol, polyether diol, polyester diol, polycaprolactone diol, and polycarbonate diol, each having a molecular weight of up to about 9000. More specifically, examples of diol include alkylene diol such as ethylene glycol, 1,2-propylene glycol, 1,3-propane diol, 1,4-butane diol, 1,6-hexane diol; alicyclic diol such as 1,2-cyclohexane diol, 1,4-cyclohexane diol; aromatic diol such as bisphenol A, bisphenol F, resorcinol, hydroquinone; polyester diol prepared by an esterification reaction between polycarboxylic acid or the anhydride thereof, and polyol, polycaprolactone diol prepared by an polymerization reaction of caprolactone by using polyol as an initiator; and polyether diol such as polyoxyethylene glycol, polyoxypropylene glycol, polyoxytetramethylene glycol, the random or block copolymer thereof.
The diol is preferably primary diol of which hydroxyl groups are both primary. The reason is that primary diol is conveniently prepared by a simple procedure as described hereinafter. Preferred example of the primary diol is an ethylene oxide adduct of bisphenol A having a structure of the formula: 
wherein x represents an integer of 1 to 10. The x in the formula is preferably an integer of 2 to 6 because both flexibility and corrosion resistance of the resulting coated film are easily achieved.
Polyurethane diol is the diol which is prepared by condensing diisocyanate with diol. The polyurethane diol may also be comprised in diol of the present invention. Preferred example of the polyurethane diol is that having a structure of the formula: 
wherein R4 represents a residue excluding hydroxyl groups of diol, R5 represents a residue excluding isocyanate groups of polyisocyanate, and y represents an integer of 1 to 10.
The diol which corresponds to R4 is not limited to, but the diol which corresponds to R4 have to be different from polyurethane diol. Preferred examples of the diol include alkylene diol, aromatic diol, polyether diol, polyester diol, polycaprolactone diol, and polycarbonate diol. It is preferred that primary diol is employed as the diol which corresponds to R4 because the resulting polyurethane diol also become primary diol. Particularly preferred example of the diol which corresponds to R4, is the above described ethylene oxide adduct of bisphenol A.
The diisocyanate which corresponds to R5 is preferably aliphatic diisocyanate. The reason is that weather resistance of the resulting epoxy resin becomes excellent for use as a coated film. Preferred examples of the diisocyanate which corresponds to R5 include hexamethylene diisocyanate (HMDI), isophorone diisocyanate (IPDI), 4,4xe2x80x2-methylenebis(cyclohexylisocyanate), trimethylhexamethylene diisocyanate.
The polyurethane diol employed herein preferably has a molecular weight of from 300 to 9000, preferably 500 to 5000. If the molecular weight of the polyurethane diol is more than 9000, appearance of the coated film becomes poor, and if it is less than 300, flexibility of the coated film becomes poor.
A blocking agent is the low molecular weight compound which is temporarily allowed to react with an isocyanate group present in isocyanate, in order to prevent an unintended reaction of the isocyanate group at room temperature. The blocking agent is generally eliminated from the blocked isocyanate group when heated.
Any compound well known to the art as a blocking agent may be employed. Examples of the blocking agent include aliphatic alcohol such as methanol, ethanol, isopropanol, n-butanol, 2-ethyl hexanol, ethylene glycol monobutyl ether, cyclohexanol; phenols and their derivatives such as phenol, nitrophenol, ethyl phenol; oximes and their derivatives such as methyl ethyl ketoxime; lactams and their derivatives such as xcex5-caprolactam. Preferred example of the blocking agent includes methanol.
The reaction of diisocyanate, diol, and a blocking agent may be conducted in the manner well known to those skilled in the art. The three compounds, for example, are charged into a suitable container at a suitable amount ratio in the presence or absence of a suitable solvent, and are heated and stirred. Blocked polyurethane diisocyanate is obtained by this reaction.
The reaction amount ratio by mol of diisocyanate, diol, and blocking agent is generally 1.0 to 3.0/0.5 to 2.0/0.5 to 2.0. The repetition unit number xe2x80x9cmxe2x80x9d in the blocked polyurethane diisocyanate and the oxazolidone ring-containing epoxy resin of the present invention may appropriately be controlled by changing the reaction amount of diisocyanate, diol, and blocking agent.
When the ethylene oxide adduct of bisphenol A is employed as the diol which corresponds to R2, m is preferred to be an integer of from 2 to 10, particularly 2 to 6. If m is more than 10, corrosion resistance of the coated film becomes poor, and if it is less than 2, flexibility of the coated film becomes poor.
When the polyurethane diol is employed as the diol which corresponds to R2, m is preferred to be an integer of from 2 to 6, particularly 2 to 4. If m is more than 6, appearance of the coated film becomes poor, and if it is less than 2, flexibility of the coated film becomes poor.
The blocked polyurethane diisocyanate is then allowed to react with diepoxide. The oxazolidone ring-containing epoxy resin (a) of the present invention is prepared by this reaction (the step by step method).
The reaction of the blocked polyurethane diisocyanate with diepoxide may be conducted in the same manner as that described in J. Polymer Sci. Part A-1, 4, 751 (1966), Iwakura et al. Basically, the two components are charged into a suitable container at a suitable amount ratio in the presence or absence of a suitable solvent, and are heated and stirred.
As the diepoxide, for example, a polyphenol diglycidyl ether type epoxy resin which is a reaction product of a polycyclic polyphenol compound such as bisphenol A and bisphenol F, with epichlorohydrin; polyglycidyl ether of polyhydric alcohol such as ethylene glycol, 1,4-butane diol, and 1,6-hexane diol; polyglycidyl ester of aliphatic, alicyclic, or aromatic polycarboxylic acid; are employed. Particularly preferred examples of the diepoxide include bisphenol A diglycidyl ether.
The reaction amount ratio by mol of blocked polyurethane diisocyanate and diepoxide is generally 2 to 10 mol of diepoxide based on 1 mol of blocked polyurethane diisocyanate. The repetition unit number xe2x80x9cnxe2x80x9d in the oxazolidone ring-containing epoxy resin of the present invention may appropriately be controlled by changing the reaction amount of blocked polyurethane diisocyanate, and diepoxide.
When the ethylene oxide adduct of bisphenol A is employed as the diol which corresponds to R2, n is preferred to be an integer of from 1 to 5, particularly 1 to 4. If n is more than 5, corrosion resistance of the coated film becomes poor, and if it is less than 1, flexibility of the coated film becomes poor.
When the polyurethane diol is employed as the diol which corresponds to R2, n is preferred to be an integer of from 1 to 5, particularly 1 to 3. If n is more than 5, appearance of the coated film becomes poor, and if it is less than 1, flexibility of the coated film becomes poor.
The reaction temperature is preferably 60 to 200xc2x0 C. The blocking agent of the blocked diisocyanate is released as the reaction proceeds. The released blocking agent may allow to present in the reaction mixture, or may be removed from the reaction mixture by using a decanter and the like.
The reaction may proceed smoothly by employing tertiary amine as a catalyst. This is advantageously applicable to the reaction of the blocked isocyanate with the polyepoxide. Examples of the tertiary amine include N,N-dimethylbenzylamine, triethylamine, N,N-dimethylcyclohexylamine, N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine, N-methylmorpholine, 1,8-diazabicyclo[5.4.0]undecene, 1,4-diazabicyclo[2.2.2]octane, pyridine, quinoline, and imidazole.
In addition to tertiary amine, a tin compound such as di-n-butyltin dilaurate, tin (II) chloride, tin octenoate, dibutyltin oxide, dioctyltin oxide, 1,3-diacetoxytetrabutyl distannoxane, 1,3-dichlorotetrabutyl distannoxane, and dibutyldibutoxytin, may be employed at once.
The oxazolidone ring-containing epoxy resin of the present invention may be prepared by more simple process. That is, diisocyanate, diol, and a blocking agent are stirred in diepoxide employed as a solvent. In this instance, the diisocyanate, and the diol, and the blocking agent are reacted to form blocked polyisocyanate by first. The above described catalyst is then optionally added, and the reaction mixture is continued to stir and heat. Thereby, the blocked polyurethane diisocyanate is reacted with the diepoxide to obtain the oxazolidone ring-containing epoxy resin of the present invention (the in situ method).
When this preparation process is employed, it is preferred to employ primary diol as the diol. If secondary diol or tertiary diol is employed, the diisocyanate resulted by the reaction further reacts not only with the hydroxyl group of the diol, but also with the hydroxyl group present in the epoxy resin. Thereby, gelation of the reaction mixture may occur, and the reaction may not suitably proceed.
It is usually possible to make the resulting oxazolidone ring-containing epoxy resin water-soluble. Water-solubilization of the oxazolidone ring-containing epoxy resin may be conducted by introducing therein an ionic group. The resulting water-soluble resin may be employed as a binder of an aqueous coating composition. For example, a terminal epoxy group is allowed to ring-open with an active hydrogen compound having an ionic group, and result in, an oxazolidone ring-containing aqueous resin in which an ionic group such as an amino group, and a carboxyl group, is introduced. Further, an aqueous coating composition, for example, an electrodeposition coating composition, may be prepared by using such an oxazolidone ring-containing aqueous resin.
If desired, the resulting oxazolidone ring-containing epoxy resin may be allowed to reacted with polyhydric alcohol, or polycarboxylic acid to extend the chain thereof. Examples of the polyhydric alcohol employed for this purpose include alkylene diol such as ethylene glycol, 1,2-propylene glycol, 1,3-propane diol, 1,4-butane diol, and 1,6-hexane diol; alicyclic diol such as 1,2-cyclohexane diol, and 1,4-cyclohexane diol; aromatic diol such as bisphenol A, bisphenol F, resorcinol, and hydroquinone; polyester diol having a molecular weight of about 300 to 3000 such as that obtained by the esterification reaction of polycarboxylic acid, or anhydride thereof, with polyol, and polycaprolactone diol prepared by the polymerization reaction of caprolactone by using polyol as an initiator; and polyether diol having a molecular weight of 300 to 3000 such as polyoxyethylene glycol, polyoxypropylene glycol, polyoxytetramethylene glycol, the random or block copolymers thereof.
The chain extension reaction may be conducted by using aliphatic dicarboxylic acid such as succinic acid, adipic acid, azelaic acid, dodecane diacid, dimer acid, long-chain aliphatic dicarboxylic acid having 18 to 20 carbon atoms, altered butadiene-acrylonitrile copolymer having a terminal carboxyl group, or aromatic dicarboxylic acid such as phthalic acid, isophthalic acid, and terephthalic acid.
When partial ring-opening of an epoxy group is desired without conducting the chain extension reaction, aliphatic primary alcohol such as n-butanol, ethylene glycol monobutyl ether, octanol, stearyl alcohol; monoalcohol such as xylenol, p-t-butylphenol, p-nonylphenol; aliphatic monocarboxylic acid such as acetic acid, lactic acid, butyric acid, octanoic acid, cyclohexane carboxylic acid, lauric acid, stearic acid, and 1,2-hydroxystearic acid; aromatic monocarboxylic acid such as benzoic acid, and 1-naphthoic acid.
When part of epoxy groups present in the oxazolidone ring-containing epoxy resin are ring-opened by using monoalcohol, or monocarboxylic acid, the chain extension does not occur. Whereas, when the ring-opening reaction is conducted by using a mixture of monoalcohol and diol, or a mixture of monocarboxylic acid and dicarboxylic acid, the ring-opening reaction and the chain extension reaction occur at once. In this instance, the monoalcohol and the monocarboxylic acid work as a termination agent of the chain extension reaction.
The active hydrogen compounds employed for the partial ring-opening reaction (e.g., monoalcohol, diol, monocarboxylic acid, and dicarboxylic acid) are hereinafter referred to as xe2x80x9cfirst active hydrogen compound (b)xe2x80x9d.
After the partial ring-opening reaction, the epoxy groups left in the oxazolidone ring-containing epoxy resin may be utilized to introduce an ionic groups into the resin. That is, the residual epoxy groups in the oxazolidone ring-containing epoxy resin may be allowed to react with an active hydrogen compound having an ionic group.
When an aqueous resin having a cationic group is desired, an acid salt of primary amine, secondary amine, or tertiary amine, or a mixture of sulfide and acid, are employed as the active hydrogen compound having an ionic group. Examples thereof include butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methylehanolamine, triethylammonium chloride, N,N-dimethylethanolammonium acetate, a mixture of diethyldisulfide and acetic acid; a secondary amine compound in which a primary amino group is blocked, such as ketimine capped aminoethylethanolamine, and diketimine capped diethylenetriamine. Plural amines may be employed at once.
Primary amine is two equivalent, and it works as a chain extending agent of a polyepoxy compound. Therefore, when primary amine is employed as the active hydrogen compound having an ionic group, the polyepoxy compound is polymerized. Thereby, a polyepoxy compound having high molecular weight is obtained. These amines for reacting with an epoxy group is preferred to be employed in an amount equivalent to the epoxy group present in the oxazolidone ring-containing epoxy resin.
The resulting oxazolidone ring-containing aqueous resin having a cationic group preferably has a number average molecular weight of from 600 to 4000 measured by the GPC method. If the number average molecular weight is less than 600, film forming property of the aqueous resin becomes poor, and when it is over 4000, emulsifying and water-solubilizing the aqueous resin become difficult. The aqueous resin preferably has an amine equivalent weight of from 0.3 to 4.0 meq/g. If the amine equivalent weight is less than 0.3 meq/g, emulsifying and water-solubilizing the aqueous resin become difficult, and when it is over 4.0 meq/g, water resistance of the resulting coated film becomes poor.
When an aqueous resin having an anionic group is desired, polycarboxylic acid such as succinic acid, maleic acid, fumaric acid, phthalic acid, adipic acid, azelaic acid, is allowed to react with an epoxy group present in the oxazolidone ring-containing epoxy resin instead of the amine. Thereby, the half esterification reaction occurs, and an aqueous resin having a terminal carboxylic group is obtained.
The resulting aqueous resin having an anionic group preferably has a number average molecular weight of from 600 to 4000 measured by the GPC method. If the number average molecular weight is less than 600, film forming property of the aqueous resin becomes poor, and when it is over 4000, emulsifying and water-solubilizing the aqueous resin become difficult. The aqueous resin preferably has an acid equivalent weight of from 0.3 to 4.0 meq/g. If the acid equivalent weight is less than 0.3 meq/g, emulsifying and water-solubilizing the aqueous resin become difficult, and when it is over 4.0 meq/g, water resistance of the resulting coated film becomes poor.
The active hydrogen compounds having an ionic group employed for the water-solubilization reaction (e.g., an acid salt of amine, and polycarboxylic acid) are hereinafter referred to as xe2x80x9csecond active hydrogen compound (c)xe2x80x9d.
The oxazolidone ring-containing aqueous resin is a reaction product of the components oxazolidone ring-containing epoxy resin (a), the first active hydrogen compound (b), and the second active hydrogen compound (c), as described above. In order to achieve high smoothness, high corrosion resistance of the coated film, and high multi-coating ability, the oxazolidone ring-containing aqueous resin contains preferably 35 to 95% by weight, more preferably 45 to 85% by weight of the component (a) based on solid.
The oxazolidone ring-containing aqueous resin obtained from the above described process may be crosslinked by using a closslinking agent such as a melamine resin, and a polyisocyanate compound. The feature of the oxazolidone ring-containing aqueous resin of the present invention is effectively exerted when it is used as a binder of an aqueous coating composition, particularly an electrodeposition coating composition.
An electrodeposition coating composition which employs the oxazolidone ring-containing aqueous resin of the present invention affords operational convenience in zinc plate coating process, and a coated film excellent in smoothness, corrosion resistance, multi-coating ability, chipping resistance, and heat resistance.
Generally, the oxazolidone ring-containing aqueous resin of the present invention, and a closslinking agent are dispersed into an aqueous medium which contains a neutralizing agent to obtain an aqueous coating composition, e.g., an electrodeposition coating composition. Examples of the closslinking agent typically include a melamine resin such as etherized methylolmelamine, and blocked polyisocyanate.
The blocked polyisocyanate means the polyisocyanate in which a isocyanate group is protected by a blocking agent. The blocking agent is the same as that described in preparing the blocked polyurethane diisocyanate. The above described diisocyanate is also included in the polyisocyanate.
Specific examples of the polyisocyanate include hexamethylene diisocyanurate, and norbornane diisocyanate.
For preparing a cationic electrodeposition coating composition, the oxazolidone ring-containing aqueous resin in which an amino group or a sulfonium group is introduced, have to be employed as an aqueous resin, and an inorganic or an organic acid such as hydrochloric acid, nitric acid, phosphoric acid, formic acid, acetic acid, and lactic acid, have to be employed as a neutralizing agent.
For preparing an anionic electrodeposition coating composition, the oxazolidone ring-containing aqueous resin in which a carboxyl group is introduced, have to be employed as an aqueous resin, and an inorganic or an organic base such as sodium hydroxide, potassium hydroxide, organic amines, have to be employed as a neutralizing agent.
The crosslinking agent have to be included in an amount sufficient for reacting with a functional group present in the resin such as a hydroxyl group to form a satisfactory cured coated film, and the amount is generally from 5 to 50% by weight based on solid. The neutralizing agent have to be included in an amount sufficient for neutralizing at least 20%, preferably 30 to 60% of amino groups or carboxyl groups present in the resin.
A tin compound such as dibutyltin dilaurate, and dibutyltin oxide, or the conventional urethane cleavage catalysts may be included in the coating composition which contains blocked polyisocyanate. The amount of the catalysts is usually 0.1 to 5% by weight of the blocked polyisocyanate.
A coloring pigment such as titanium dioxide, carbon black, colcothar, a rustproofing pigment such as basic lead silicate, aluminium phosphomolybdate, a body pigment such as kaolin, clay, and talc, and a conventional additives for an aqueous coating composition such as an water-miscible organic solvent, a surface active agent, an antioxidant, and a UV absorber, may be included in the coating composition.
The electrodeposition coating method is particularly suitable for coating the aqueous coating composition of the present invention. However, the other coating method such as the dip coating method and the spray coating method may also be employed.
The following examples further illustrate the present invention, however, these are not to be construed as limiting the present invention to their details. The xe2x80x9cpartsxe2x80x9d, and the xe2x80x9c%xe2x80x9d in the examples are the values based on weight unless otherwise indicated. The xe2x80x9cepoxy equivalent weightxe2x80x9d and the xe2x80x9camine equivalent weightxe2x80x9d in the examples are the values based on solid unless otherwise indicated.